As shown in FIG. 1, in an isolated type power supply 10, a diode bridge 12 and a capacitor 14 are used to rectify and filter the alternating-current (AC) input voltage VAC to generate a direct-current (DC) input voltage Vin applied to a primary coil Lp of a transformer 18, a power switch 20 is serially connected to the primary coil Lp to switch the current Ip of the primary coil Lp by a control signal Vgate for power delivery to a secondary coil Ls of the transformer 18 to thereby generate an output voltage Vout at an output terminal, a current sense resistor Rcs is serially connected to the primary coil Lp to detect the current Ip, and a power controller 16 determines the duty of the power switch 20 according to the voltage Vcs established by the current sense resistor Rcs and the voltage Vcomp fed back from the output terminal Vout to generate the control signal Vgate to switch the power switch 20. When the current sense resistor Rcs becomes short circuit, the voltage Vcs becomes zero, and the duty of the power switch 20 will be set to its maximum, causing the current Ip rising steeply. The large current Ip will push the transformer 18 into saturation and may burn out the current sense resistor Rcs, the power switch 20 as well as the power controller 16. Therefore, there is need of detection for short circuit of the current sense resistor Rcs to turn off the power switch 20 for protecting the power supply 10.
Conventionally, the protection mechanism monitors the voltage Vcs and triggers a warning signal if the voltage Vcs is still lower than a reference voltage after a period of time in a pulse width modulation (PWM) cycle. If the warning signal has continuously appeared in several successive cycles, for example, eight successive cycles, the current sense resistor Rcs will be identified as being short circuit, and the isolated type power supply 10 will enter a protection mode. However, in the case where the input voltage Vin is high, the current Ip increases very steeply after the current sense resistor Rcs becomes short circuit, which may place the isolated type power supply 10 under overcurrent condition before the isolated type power supply 10 enters its protection mode.
Therefore, it is desired a protection apparatus and method for quick detection for short circuit of the current sense resistor of an isolated type power supply for protecting the isolated type power supply.